The invention relates to correcting register faults in a multicolor printing machine having a number of items of equipment for the digital production of color separations, correction values being determined and assigned to the angular positions of at least one image cylinder and, as a result, being taken into account for controlling the production of color separations on such image cylinder.
Printing colored illustrations, in particular colored images, may be carried out by a number of color separations being printed over one another. These are generally the colors yellow, magenta and cyan as well as black. If required, special colors are added. By overprinting these colors, all color compositions can be achieved, the quality of the prints depending significantly on the in-register overprinting of the color separations. In the case of digital printing processes, for example electrostatic printing processes, the maintenance of the register of the overprint is achieved by the image production devices being controlled in such a way that the color separations meet one another in-register when the are transferred to a printing substrate.
U.S. Pat. No. 5,287,162 discloses a method and apparatus in which calibration tables are drawn up which contain correction values for setting images on the image cylinders. In order to draw up these calibration tables, register marks are printed by the various inking units and the times are registered which the register marks need from their production until their registration. Times of this sort are then used to determine correction values, which are assigned in the form of calibration tables to the angular positions of the image cylinders.
However, registering faults in this way by register marks has the disadvantage that the sum of all the faults is registered and, as a result, the individual fault sources can no longer be assigned to their periodic repetitions. Register faults are predominantly caused by non-roundness of machine elements that carry images or substrates. If non-roundnesses of other machine element, such as those of image transfer cylinders or of the drive roller of the carrier of printing substrates, are added to the non-roundnesses of the image cylinders, these can no longer be assigned directly to the angular positions of image cylinders by the aforementioned prior art, since these machine elements do not rotate absolutely simultaneously with the image cylinders. Such a simultaneous rotation is lacking even if the diameters are coordinated with one another, since phenomena such as slippage and overdrive occur. Overdrive is the name give to the more rapid rotation of cylindersxe2x80x94in which one cylinder drives the otherxe2x80x94as related to rigid cylinders rolling on one another. This is caused by the compression of an elastic cover in the area of contact between the cylinders. Slippage can primarily occur when one machine element is driven by another, for example an image transfer cylinder by the carrier for printing substrates and such carrier in turn by the drive roller. The image cylinder can also be driven by the carrier or by the image transfer cylinder. In relation to the faults which repeat periodically with the rotation of a machine element, the registration of register marks also registers non-periodic changes, such as drifting, for example, as a result of temperature changes, although their assignment to periodic angular positions is not given. This in turn makes the analysis of the faults registered in total more difficult.
In addition, the printing of register marks is complicated, particularly since, for the assignment to angular positions, the continuous printing of register marks is necessary, comprising all the angular positions and angular position combinations of all the machine elements that influence the register. In addition, the accuracy of the calibration tables assigned to the angular positions is limited by the resolution accuracy of the printed register marks, and therefore precise correction for finely subdivided areas of the color separations is not possible.
The invention is therefore based on the object of making available correction values for correcting register faults which, with a fine graduation, can be assigned to angular positions of machine elements in a repeatable way.
With respect to the method, the object is achieved according to the invention by circularity errors of machine elements which have an influence on the register, being determined and assigned to their angular positions and, from them, the correction values for the control of the production of the color separations on the at least one image cylinder being determined.
With respect to the apparatus, the object is achieved according to the invention by the correction values being determined on the basis of circularity errors of machine elements which have an influence on the register, by the at least one controller being such that, by assigning the circularity errors to the angular positions of the machine elements, it determines the correction values referred to the angular positions of the at least one image cylinder for the control of the production of the color separations on the at least one image cylinder.
The advantage of the invention resides in the fact that the primary fault sources, which are based on non-roundnesses, can be assigned exactly to the machine elements, which have these non-roundnesses. In this way, the faults, with their periodic repetition with the angular positions of the relevant element, can be registered, and it is possible to control the setting of images on the image cylinders in such a way that these faults are compensated for. If, therefore, image starts or areas of a color separation are produced on the image cylinder, then the controller of the image production xe2x80x9cknowsxe2x80x9d the faults together with their periodicities bound to the relevant machine elements, and can control the image production in such a way that the faults are already counteracted at the primary stage of image production by compensation, for example by stretching or compressing image components. During the production of an image, so to speak xe2x80x9cfaultsxe2x80x9d are incorporated which cancel out the transfer faults in such a way that a color separation which is in-register in all the sub-areas of the image is printed, and the various color separations arrive on the printing substrate in-register and correctly. The invention permits the pre-calculation of the faults, the fault sources with different periodicities being registered separately and therefore the calculation of the aforementioned image-setting with fault compensation being possible.
At the same time, the invention does not require the printing of register marks, since the non-roundnesses can be registered in another way, specifically in such a way that no confusion with faults with different periodicities or with faults with no periodicity occurs. Of course, the printing of register marks, the registration of image starts or image components can be carried out as well in order to make further corrections. At the same time, the advantage of the classified registration of faults with different periodicities is nevertheless maintained, since these faults can be compensated for before printed register marks are registered.
The transmission paths, which are provided for the color separations, are of no consequence for the invention. The equipment for producing the color separations can produce the latter on a single image cylinder or on an image cylinder in each case. The transfer of the color separations can then be carried out from the one image cylinder or the plurality of image cylinders directly onto the substrate, or further transfer elements can be provided, which transfer the already superimposed or the individual color separations separately. It is also possible for the color separations already to be superimposed on an image cylinder or to be superimposed only as they are transferred to the printing substrate or on the way to the latter on a transfer element.
The configuration of the invention with respect to the circularity errors to be taken into account depends on these transmission paths and transmission elements.
Provision can therefore be made, in order to determine the correction values, for the circularity errors of the image cylinders to be determined. For multicolor printing machines with image transfer cylinders, provision can be made, in order to determine the correction values, for their circularity errors to be determined as well and therefore taken into account for the control of the production of the color separations on the image cylinders. Furthermore, the circularity errors of the drive roller of a carrier for printing substrates can also be registered and taken into account when determining the correction values for the control of the production of the color separations on the image cylinders. With respect to the apparatus for implementing these method steps, sensors for registering these circularity errors and at least one controller, which calculates the correction values, are then provided. Specific configurations will be discussed further.
There are various possibilities for determining the circularity errors. Provision can be made for the circularity errors to be determined by direct measurement on the relevant machine element. However, it is also possible for the angular positions of the machine elements which have an influence on the register to be registered, and the circularity errors to be determined from the mutual assignment of the angular positions. With respect to the apparatus, provision is then made for devices tc determine the circularity errors and assign them to the angular positions. These may be sensors for registering circularity errors, or it is possible for the devices to be rotary encoders which are connected to a computer, which determines the circularity errors and therefore the correction values from the mutual assignment of the angular positions of the machine elements. Since the circularity errors are registered with the associated angular positions, these are the effective circularity errors with the incorporation of slippage, overdrive or similar phenomena.
The correction values can be provided merely for the image starts; in order to achieve a high precision in the prints, however, it is proposed that the correction values be assigned to defined areas of the color separations and taken into account for their production. The color separations may be individual image lines or groups of image lines. Provision is preferably made for the defined areas of the color separations to be determined by assigning image areas to defined angular sequences of the image cylinders. With respect to the apparatus, provision is made for the at least one controller to be designed in such a way that it takes into account the correction values for defined areas of the color separations during their production.
With respect to the correction values, provision may be made for them to be registered once and taken into account as machine-specific parameters. For this purpose, with respect to the apparatus, provision is made for the at least one memory to contain the circularity errors with their assignment to the angular positions as machine-specific nominal values.
The correction values can also be registered before a print is made and taken into account for this print. In this case, this may be a re-determination of the correction values or it is possible to make corrections of the machine-specific parameters. Of course, the correction values can also be registered and taken into account continuously. By continuous re-determination of correction values, it is in particular also possible to take into account drifting of the values, which can occur for example as a result of changes of temperature and stresses in the machine, by the correction values being updated continuously.
In order also to be able to make corrections over the image width, provision can be made for a number of correction values to be registered over the area of the image width and taken into account. However, it is also possible for a number of correction values to be registered over the area of the image width, and for an average correction value to be determined from these and taken into account. With respect to the apparatus, an appropriate arrangement of sensors and an appropriate design of at least one controller is needed for this purpose.
The above-mentioned registration of the data and determination of correction values for the control already leads to a good result and to a high maintenance of register without being checked by the printing of register marks or similar control fields. This is therefore particularly advantageous since register corrections of this type can be carried out very quickly. Of course, the invention does not exclude the correction values being checked by printing and registering color separations and, if necessary, corrected. Here, these color separations may be test prints of image starts or test prints of further defined areas of color separations. Of course, register marks can also be printed as test prints of color separations and evaluated appropriately. With respect to the apparatus, provision is then made for at least one sensor to register test prints, and an evaluation device which evaluates test prints, for example register marks, in order to check and correct the correction values.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.
Of course, appropriate configurations of the apparatus can be assigned to the above-mentioned features described in terms of the method, and vice versa.